Impact resistant coatings for nickel-base and cobalt-base superalloys and the like

ABSTRACT

In the production of impact and oxidation resistant nickel-base metal coatings on heat resistant superalloys by pack aluminizing in which nickel aluminide is formed to provide the protective coating, markedly improved ductile coatings are produced of high impact resistance by controlling the amount of aluminum in the pack which also contains about 5 to 40 percent by weight of chromium, the amount of aluminum not exceeding 3 percent by weight of the pack.

United States Patent '91 Stueber et a1.

IMPACT RESISTANT COATINGS FOR 1 51 Feb. 13, 1973 4] I 3,257,230 6/1966 Wachtell et a1. ..117 107.2 NICKEL-BASE AND COBALT-BASE 3,073,015 1/ 1963 Wachtell et a1 ..1 17/ 107.2 X SUPER ALLOYS AND THE LIKE 3,096,205 7/1963 De Guisto ..117/107.2

3,436,249 4/1969 Lambert et a1. 1 17/130 X [75] Inventors: Richard J. Stueber, Suffern, N.Y.;

Stanley Rwerdale Primary ExaminerRa1ph S. Kendall [73] Assignee: Chroinalloy American Corporation, Attorne'ySandoe, Hopgood and Calimafde Orangeburg, N.Y. [22 Filed: Aug. 19, 1970 [57] ABSTRACT In the production of impact and oxidation resistant 21 A l.N 65 147 l 1 pp 0 nickel-base metal coatings on heat resistant superal- 1 loys by pack aluminizingin which nickel aluminide is [52] US. Cl. ..l17/107.2 P, 117/131, 29/197 f d to id h protective coating, markedly [51] I111. Cl ..C23c 9/02 improved ductile coatings are produced of high Fleld of Search p resistance y controlling the amount of aluminum in the pack which also contains about 5 to 40 percent [56] ngferences Cited by weight of chromium, the amount of aluminum not UNITED STATES PATENTS exceeding 3 percent by weight of the pack.

3,544,348 12/1970 Boone et a1. 17/1Q7.2 P 3 Claims, 1 Drawing Figure Q mere-05:0 I Y a 2 "a am t L 87040 QANG! of F 9 4! a I 0 A I I I 1 M 2 1 J 4 a 6 7 a? IMPACT, RESISTANT COATINGS FOR NICKEL- BASE AND COBALT-BASE SUPERALLOYS AND I THE LIKE sistance to impact atambient and elevated temperatures.

BACKGROUND OF THE INVENTION Metallurgical developments in recent years have indic-ated the necessity of using high nickel and high cobalt heat resistant alloys, e.g. superalloys having desirable physical properties forvarious high temperature applications, such as, for example, the manufacture of rotor blades and stator vanes for high temperature gas turbines where operation without failure is desired of the part, such as during prolonged exposure to temperatures well above 1,500F and even substantially above the temperature range at which failure or diminution of the strength characteristics may be expected of even high temperature austenitic or nickel chromium steel.

The use of supe'ralloys by themselves with nothing more have not always provided the necessary resistance to hot corrosiondamage at su'chelevated temperatures. Thus, corrosion resistant coatings have been resorted to as one means ofsfurtheraugmenting the resistance of the substrate to high temperature corrosion, particularly on complex shaped components used in contemporary jet engines where handling and gauging damage have been known to cause premature failure of protective coatings which tend to be brittle in nature A wide variety of different processes and techniques are known forproducing a diffusion coating'or layer into or on the surface of metal articles. Some of these well-known techniques (and those generally of the character towhich this invention particularly relates) involved embedding the article to be coated (or otherwise covering the surface thereof) in powdered coating pack including a powdered source of the coating material (with or without admixture with powdered inert filler) and a vaporizable halogen carrier ingredient (such as a heat-volatile halide), and heating the thus embedded article and pack in a sealed retort (or other controlled, and generally non-oxidizing atmosphere)'to an elevated temperature at which the carrier material vaporizes and/orotherwise reacts with or functions as a carrier for transporting the coating material from or throughthe powderedpack to the surface of the article to be coated for diffusion or other reaction thereat.

Generally speaking, the various chemical reactions involved (e.g., between the carrier and the coating material, and the metal or other components of the article to be coated, among whatever ingredients are in the pack, between coating'material and whatever intermetallics or alloys may have already been formed at or in the surface of the article being coated, etc.) occur more or less simultaneouslyduring the heating treatment and are mostly of a reversible nature, so that the net result of the coatingstep andthe chemical reactions therein 'involvedmay depend predominantly upon the various equilibria achieved. That is, as will be understood, under certain temperature conditions and with certain reactive carrier materials, ingredients in the coating pack may be inclined to combine with each other at the same time (and, perhaps, evenat the same rate) as one or another thereof may diffuse into the surface of the metal article; while (if the equilibrium conditions are appropriate) some portion of the metal from the article itself, or one of another component thereof, may also diffuse out of the article and into the pack ingredients.

RELATED U.S. PATENT In U.S. Pat. No. 3,257,230 assigned to the same assignee, it is stated that aluminum diffusion into the surface of a predominantly nickel-containing article may be productive of a variety of different nickel aluminides, with the particular one formed being perhaps a function of the proportion of aluminum carried to the article surface or diffused therein at the particular operating temperature. If the particular aluminide desired is one containing less than the maximum amount of aluminum, formation thereof may not occur if aluminum from the pack be too rapidly presented to or available in the article surface. If the treatment is maintained at a sufficiently high temperature and prolonged to achieve a desired thickness of coating layer, too rapid transfer of aluminum from the pack to the surface of the article (or too slow diffusion from the surface on inwardly of the article) may occur to form a lower melting high-aluminum aluminide rather than the one desired.

It was discovered that the foregoing could be avoided by including a portion of a buffering material, such as chromium, in the pack to form with the aluminum therein a preliminary intermetallic, the diffusion of which into the surface of the article can only occur at a diminished rate (or, perhaps, cannot occur at all) at the desired treating temperatures, the availability or transfer of the aluminum component in the pack for diffusion'into the surface of 'the article is readily. inhibited or controlled so that the temperature levels or other thermodynamic conditions necessary to break down the preliminary chromium aluminide sufficiently for aluminum to be diffused will produce the desired conditions for the formation of the particular nickel aluminide desired in the surface of the article.

Utilizing an inhibiting or rate controlling component such as chromium in an aluminum pack for preliminary formation therein of a chromium aluminide has been calcia, silica, zirconia and the like. It was noted that the ratio of chromium to aluminum in thepack may range up to 8 and, more preferably, from about 0.5 to 4.6

(e.g. 2 to 4). A composition found particularly'satisfactory for commercial use is one containing by weight 69% alumina as the inert diluent, 22% chromium, 8% aluminum and l% ammonium fluoride, with the coating layer or casing depths of the order of about 0.0019 inch to0.0022 inch. 1n producing the coating, the pack is enclosed in a sealed container and heated at between 1,800 to 2,lF for 4 to hours, with the temperature and time ranging more broadly from about 1,400

to 2,200F for A to 40 hours. Severe oxidation testing at 2,000F in an oxidizing atmosphere provided in a standard testing furnace indicated no failure of any of the coated parts in over 85 hours of testing treatment.

While markedly improved resistance to oxidation at elevated operating temperatures was obtainable due to the nickel aluminide formed in the coating by the foregoing process, the coatings did not havethe desired resistance to impact. Care had to be exercised in the handling of coated parts during assembly-line production. Impact-resistant coatings are required on current turbine engine hardware of complex geometries to insure that protection of the base metal is not lost as a result of chipping or spalling due to in-shop handling on the production line. Thus, a diffusion heat treatment is generally required to improve the impact resistance of the coating. However, even then failure might occur during handling of the coated part prior to heat treatment.

OBJECTS OF THE INVENTION It is thus an object of this invention to provide a pack-aluminizing process for producing ductile, hot corrosion resistant nickel aluminide coatings which do not require a diffusion heat treatment for improving the impact resistance thereof.

Another object is to provide a pack aluminizing process for producing a nickelaluminide coating of uniform thickness capable of sustaining severe plastic deformation, such as that resulting from impact without cracking or spalling.

Still another object is to provide a pack aluminizing process for producing a ductile nickel aluminide coating which will permit the base metal substrate to be strained during use into its plastic range while undergoing heavy loading or thermal stress without the coating cracking or spalling.

These and other objects will more clearly appear from the following disclosure, the appended claims and the accompanying drawing which is a graph illustrating the markedly improved resistance to impact obtained with the novel method of the invention.

' halogen transport agent or a carrier, the improvement residing in controlling the amount of aluminum in the pack over the range of about one-eighth to not more than 5 percent by weight of the pack. By maintaining the aluminum over the foregoing range and, more advantageously, over the range of about 1% to 2.5 percent, a ductile protective metal coating is assured characterized by improved resistance to impact at room and elevated temperatures,

The coating produced by the aforementioned process has a uniform thickness and can sustain severe plastic deformation, such as that resulting from impact, without cracking or spalling in such a way as to affect adversely the sulfidation and oxidation resistance of the coating. The ductility of coatings produced in accordance with the invention permits the base metal substrate to be strained into its plastic range while undergoing heavy loading or thermal stress with the result that the coating does not crack or spall.

A wide range of nickel-base alloys can be coated using the foregoing method. A typical alloy composition range is one containing by weight of up to about Cr, e.g. 5% to 30% Cr, up to about 20% ofa metal from the group consisting of Mo and W, up to about 10% of a metal from the group consisting of Cb and Ta, up to about 1% C (preferably up to about 0.5%), up to about 10% by weight of a metal from the group consisting of Ti and Al, the total amount of Ti and Al not exceeding about 12%, up tO about 20% Fe, up to about 2% Mn, up to about 2% Si, up to about 0.2% B, up to about 1% Zr, up to about 2% Hf, and the balance at least about by weight of nickel.

Examples of known alloys falling within the aforementioned range are nickel-base alloys referred to by designations Mar-M-246, lN- 738, IN-792 Udimet 500, Mar-M-432, ln-7l3, Mar-M-200, 8-1900, TRW- 6A, lN-600 and Udimet 700, and cobalt-basealloys,

such as WI-52 and Mar-M-509. Compositions of the foregoing illustrative alloys are given in Table l as follows:

TABLE 1 Chemical composition, weight percent Ni 00 M0 W Cb Fe Ti Al B Zr Others Alloy designation C Cr IN-738 0. 13 15. 0 IN-792 0.21 12. 7 lN-713C. 0. 12 12. 5 IN600 0. 04 15. 8

Udimet 500 0.08 18.0 0. 08 15. 0 0. 15 9. 0 O. 15 0. 0 0. 15 15. 4 0. 10 8.0 0. 13 6. 1

Mar-M409 0.6 21.0

10.0 Bal. 7.0 1.5

ln carrying out the pack cementation process, a set of doublenested retorts is used in which the inner and outer retorts are glass sealed to inhibit the entry of air during the coating process. However, single retorts may be used as well. The pack is freshly prepared and then prereacted by heating the pack to a temperature of, for example, 1,800" to 2,200F for about 1 to 20 hours, the pre-reacted powder being then screened and used as the pack into which the article to be coated is then embedded. The pack composition may range by weight from about 5 to 40 percent chromium, or to 30% chromium, about one-eighth to not more than 5% aluminum, 'a small but effective amount of a halogen material-energizer (e.g. from about one-eighth to l or 2 percent) and the balance a diluent material, e.g. such refractory oxides as alumina, zirconia, calcia, silica, and the like.

DETAILS OF THE INVENTION Typical pfid'r'inaieriiisifipio ed in making up the pack are given by way of example as follows:

Pack Constituent if i I Function Examples of other energizers are NH I, NH Br, NH Cl, and the like. Broadly speaking, the diffusion coating treatment is carried out at a temperature of about l,400 to' 2,200F for about %to 40 hours. A

. more preferred range is l,750 to 2,050F for about 10 to 30 hours. A pack cementation treatment found particularly advantageous in coating alloys of Table l is l,900F for about 25 hours at temperature, using a blended powder pack composition containing chromium, 2% aluminum, about ammonium bifluoride and the balance essentially alumina.

In determining the impact resistance of a particular coating,a drop weight test is employed in which a metal weight (e.g..a half-pound weight) with a projection or indenter having a 0.065 inch radius at its impact end is dropped upon a coated test piece from various heights measured in inches, the impact value being determined in inchpounds at. the point where the coating fails by cracking orspalling. Alloys coated from a pack containing by weight 20% chromium, 3% aluminum, ammonium'bifluoride and the balance essentially alumina resulted in impact values (ductile coating) of over 17 inch pounds (high impact resistant), while thesamepack containing 8% by weight of aluminum resulted in inferior impact values (brittle coating) as low as 3 inch pounds.

The improved ductility of the coating provided by the invention enables the coating to deform as the base metal substrate is strained due to applied load orthermal stress arising in the environment of use, such as airfoils, in jet engines. This ability of the coating to deform with fracture with applied stress, such as might arise from thermal shock, protects the-base metal against catastrophic oxidation or hot corrosion. In the case of the more brittle nickel or cobalt aluminide coatings produced outside the invention, load-induced strain of the base metal in the elastic range resulted in severe cracking of the coating.

As illustrative of the preferred embodiment of the invention, the followingexample is given.

Example 1 V A series of pack cementation compositions was tested in which the aluminum in the pack was varied from about one-eighth to 8 percent by weight with the chromium level at about 20 percent. The pack also contained about NH FHF and the balance alumina. The tests were carried out on a series of commercial nickel-base alloys, the compositions of which are given in Table 1. Test pieces of each alloy were em bedded in the pack in a sealed retort which pack had first been pre-reacted (as described hereinbefore) and the coating step carried out at 1,900F for 25 hours. Following the coating, the test pieces were brushed clean and then subjected to the impact test described hereinbefore. The resulting coatings varied in thickness from about 1.2 to 5 mils (0.0012 to 0.005 inch) and seemed to relate directly to the amount of aluminum powder'in the pack. Thus, the thinnest coating was obtained at the lower range, while the 5 mil thickness was obtained with the pack containing 8% aluminum. The coatings which exhibited optimum impact values of over 17 inch pounds and which were ductile were obtained in pack compositions containing not more than 5 percent aluminum. At aluminum contents in the pack of over 5 percent, that is, over 6, 7 and up to 8 percent, the coatings were brittle and exhibited impact values as low as 3 inch pounds. The results obtained in the aforementioned tests are given in Table 2.

To determine whether the ductility of the coating was a function of the case depth of the coating per se, the following experiment was performed. The impact specimen was coated in a powder mixture containing 20"/o Cr, 3 /o Al, Ai /o NH FHF and the balance A1 0 and the specimen removed from the powder pack and impact tested. The impact value was greater than 17 inch pounds. The same specimen was subsequently coated again in a powder mixture of the same composition, removed and impact tested. The result was an impact value of greater than 17 inch pounds. Metallographic inspection of the coating on this specimen revealedv a case depth of 5.1 mils (0.0051 inch).

The unexpected result obtained with packs containing one-eighth to not more than 5% aluminum was not recognized in US. Pat. No. 3,257,230. In fact, the patent states at column 11, lines 13 to 17, that, for its purposesra pack compositionof 69% alumina, 22% chromium, 8% aluminum and 1% ammonium'fluoride was satisfactory. Thus,-the. patent is not aware that ductile. coatings can be obtained provided that the aluminum in the pack does not exceed 5 percent.

TABLE 2 coatings on nickel base superalloys Pack Composition (w/o Al) (20 CrX Al-l/4 NILIFIIF-Bfll. A1

was found that the coated alloy could withstand a total strain, i.e. plastic deformation, of up to 0.3%. Additional testing has shown that the coating produced in Alloy 1/2 3/4 1.5 2 3 5 8 MarM-246...- .0* D/2.5 D/2.8 D/2.6 :0 25 D/3.0 D ao B/5.0

.5 D/2.5 D/2.5 D/2.5 D/2.5 D/3.0 D/3.0 B/5.0 D/1.5 D/2.0 D/1.5 D/l.8 D 2.0 B/4.5 .o D/3.5 D/2.5 n ao D/2.5 D/3.5 .5 D/2.5 D/2.3 D/2.5 D/2.5 D/2.5 D/2.5 .5 D/2.5 D 2.0 D/2.2 D/3.5 D/3.5 D/3.5 B/5.0 .5 D/1.9 D/1.8 D/2.0 D/2.0 D/2.0 D/2.8 B1900 .0 D/2.5 D/2.3 D/1.8 D/3.0 D/3.0 13/50 "FEW-6A .2 D/1.5 D/1.5 D/1.4 D/2.0 D/2.0 D/2.5 B/5.0

*D=Ducti1e impact values greater than 17 inch pounds. "B=Brittlo impact values less than 17 inch pounds. I NOTE .Coating thickness is given in mils (one mil=0.001).

si il tests d d using a k tai i accordance with the invention exhibited good oxidachromium, /4% NH FHF, from 1 to 3% aluminum tion resistance in excess of 400 hours at 2,000F. Good d h b l l i whil l half as m h sulfidation and hot corrosion protection were indicated chromium was used in the pack, good ductile coatings 20 at the Same mperat re far in excess of 100 hours. I were obtained exhibiting optimum impact values of The P composition may range -by weight broadly The values of over 17 inch pounds reported for the from about 5 Percent to 40 Percent chromium, about tests on coatings within the invention means that no 175% to more: than 5% aluminum, about to 1 1 spalling or cracking occurred when the half-pound 2% hahde ohetglzet -gabout 4 and the w i ht was did f m a h ight f 34 in h ,5 x 34 balance essentially a refractory oxide diluent, e.g. alu- 17 i h d h li i f h apparatus However, mina. Within the foregoing ranges, the chromium may at an aluminum composition in the pack of 8 percent, be controlled from about 7 to percent and the alu- I the coating was very brittle and gave a reading of 3 inch ththum from about 1/5 Pa to 215%- we have found pounds, thus indicating that the coatings of the inven- It advantageous m Wotkthg over the foregoing ranges to i are over fivetimes bettcr i f as impact 30 employ a chromium to aluminum ratio of about 4:1 to sistance is concerned using the drop test as the V 160:1 and, more advantageously, about 8:1 to 100:1. criterion. Although the present invention has been described in Using a one and one ha|f pound weight (1% lb) cop conjunction with preferred embodiments, is to be unresponding to a total impact from a maximum height of derstood t modtficatttfns and vattatfohs may be 34 ow inch pounds m X 34 the 35 strate ists tats-"t. ittatfssttntzssa Shown m the attached drawmg was obtained on the understand. Such modifications and variations are concoated alloy deslsnated as (note Table sidered to be within the purview and scope of the in- The pack was varied from alummum up to 8% aluvention and the appended claims 'mmum, the pack also containing 20% chromium, {4% 40 what is claimed is: of NH Fi lF and the balance alumina. The alumimzmg A method of producing aluminized coatings on v was earned out at 19000}: for hours- Between ahout nickel-base heat resistant alloys characterized by mar 1/5 to 2% Percent aluminum the P p y kedly improved ductility under impact which com- 7 between about a to 2%%), failure at impact (that is, prises;

cracking of the g) began at the relatively g providing said alloy having a composition containing level of about 35 inch pounds (optimum impact reb i h up to about 30% C up to about 20% f sistance) with the impact value beginning to fall off at a metal f m h group consisting f Mo and w up above 26% aluminum and thereafter at a fairly rapid to about 10% of meta] f the group consisting rate at just below 5% aluminum pack concentration. f Cb d T up to about 1% C, up to about 10% From below 5% aluminum to about 6 percent and 7% by weight of a metal from the group consisting of aluminum pack concentration, the falling off in impact Ti and Al, the total amount of Ti and Al not exresistance is sharp and reaches a very low level of ceeding about 12%, up to about 20% Fe, up to between 2% to 3 inch pounds at 8% aluminum concenabout 2% Mn, up to about 2% Si, up to about 0.2% tration. Thus, below 5% aluminum concentration, the B, up to about 1% Zr, up to about 2% Hf, and the coating has good ductility and exhibits relatively high balance at least about 45% by weight of nickel; impact values, the optimum impact values being preparing an aluminum-containing pack consisting achieved about 292% aluminum concentration and entially by weight of about 5 to 0% r u below, that is, advantageously from about a to 2%% n effective amount of aluminum not exceeding 3 aluminum c n tration, percent, a small but effective amount of a halide Room temperature tensile testing of a coated g z and the balance an ne uent, 1N-713C alloy'(coated in a pack of 20% Cr'-3%-Al-V4% Prereacting Said P y heating Said P at a NH,FHF and Al,0;,,the balance), showed that no Petatul'e g g from about 1,3000}: to ki occurred when the alloy was loaded and embedding a nickel-base heat resistant alloy in said strained throughout the elastic range when viewed Prereaoted P under a microscope at a magnification of 500 times. it 5 and carryingtout Said etho at an elevated aluminizing temperature with the amount of aluminum in said pack less than 3 percent, the ratio of chromium to aluminum in said pack being in ex-' 9 10 t cess OM51, I 286% and the inert diluent is a refractory oxide.

whereby coatmg as mckel m and 3 The method of claim 1, wherein the ratio of having markedly mp ductlmy oblamedchromium to aluminum in the pack ranges from above 2. The method of claim 1, wherein the chromium to 160:1. content of said pack ranges by weight from about to 5 30%, the aluminum content ranging from about 12% to 

1. A method of producing aluminized coatings on nickel-base heat resistant alloys characterized by markedly improved ductility under impact which comprises: providing said alloy having a composition containing by weight, up to about 30% Cr, up to about 20% of a metal from the group consisting of Mo and W, up to about 10% of a metal from the group consisting of Cb and Ta, up to about 1% C, up to about 10% by weight of a metal from the group consisting of Ti and Al, the total amount of Ti and Al not exceeding about 12%, up to about 20% Fe, up to about 2% Mn, up to about 2% Si, up to about 0.2% B, up to about 1% Zr, up to about 2% Hf, and the balance at least about 45% by weight of nickel; preparing an aluminum-containing pack consisting essentially by weight of about 5 to 40% chromium, an effective amount of aluminum not exceeding 3 percent, a small but effective amount of a halide energizer, and the balance an inert diluent, prereacting said pack by heating said pack at a temperature ranging from about 1,800*F to 2,200*F, embedding a nickel-base heat resistant alloy in said prereacted pack, and carrying out said method at an elevated aluminizing temperature with the amount of aluminum in said pack less than 3 percent, the ratio of chromium to aluminum in said pack being in excess of 4:1, whereby a coating containing nickel aluminide and having markedly improved ductility is obtained.
 2. The method of claim 1, wherein the chromium content of said pack ranges by weight from about 10 to 30%, the aluminum content ranging from about 1/2 % to 2 1/2 % and the inert diluent is a refractory oxide. 